1. Field of the Invention
This invention relates to a lead-frame and, more particularly, relates to a multi-layer lead-frame for a resin-sealed type semiconductor device which includes a signal layer, at least two metal plates or planes, and a hermetically sealed resin.
2. Description of the Related Art
A conventionally known multi-layer lead-frame for a semiconductor device comprises at least three conducting layers, i.e., a signal layer, a power supply layer and a ground layer, which are affixed or laminated to each other, in turn, by means of electrically insulated films made of, for example, polyimide.
In such a multi-layer lead-frame, if a decoupling capacitor is disposed between the power supply layer and the ground layer, a kind of electrical noise, called "bounce", can be reduced and therefore the change or variation of the electrical voltage in the power supply system or the ground system due to switching operation of the semiconductor chip can be advantageously minimized.
Using such a multi-layer lead-frame enables the mounting of a relatively high-power semiconductor chip on the metal plane, due to the heat radiation from the metal plane. Also, the metal plane can be used as a ground layer or a power supply layer to improve the electrical characteristics of the semiconductor device.
Recently, however, since high-speed electronic devices have become more and more in demand, a voltage reduction can occur in the power supply and ground systems due to the simultaneous switching of the input and output buffers. This would lead an error in operation.
In order to solve the above problems and improve the electronic device, it has been proposed that the electrostatic capacitance is increased. In Japanese Unexamined Patent Publication (Kokai) No 4-73825, an insulation sheet is disposed between the power supply layer and the ground layer. Such an insulation sheet is made from an insulation resin that contains a ferroelectric substance such as tantalate oxide powder or barium titanate powder, to increase the electrostatic capacitance of the decoupling capacitor.
However, such a multi-layer lead-frame, known in the prior art has the following problems. That is to say, although a ferroelectric powder was merely contained in the insulation resin, it has been found that an expected theoretical value of electrostatic capacitance could not be obtained, even though the reasons are not clearly understood. For example, an insulation sheet containing barium titanate powder has a dielectric constant (.epsilon.) of about 10.
As a decoupling capacitor having a large electrostatic capacitance, such as 10 nF to 50 nF, is required, the above-mentioned prior art cannot fully satisfy such a requirement.
To obtain a decoupling capacitor having a larger electrostatic capacitance, a large amount of ferroelectric powder could be contained in the insulation resin but the adhesive characteristic of the insulation resin would be significantly be reduced and therefore such a decoupling capacitor could not be practically used.